Sanitary paper tissue products are widely used. Such items are commercially offered in formats tailored for a variety of uses such as facial tissues, toilet tissues and absorbent towels. The formats, i.e. basis weight, thickness, strength, sheet size, dispensing medium, etc. of these products often differ widely, but they are linked by the common process by which they originate, the so-called creped papermaking process.
Creping is a means of mechanically compacting paper in the machine direction. The result is an increase in basis weight (mass per unit area) as well as dramatic changes in many physical properties, particularly when measured in the machine direction. Creping is generally accomplished with a flexible blade, a so-called doctor blade, against a Yankee dryer in an on machine operation. This blade is also sometimes referred to as a creping blade or simply a creper.
A Yankee dryer is a large diameter, generally 8-20 foot drum which is designed to be pressurized with steam to provide a hot surface for completing the drying of papermaking webs at the end of the papermaking process. The paper web which is first formed on a foraminous forming carrier, such as a Fourdrinier wire, where it is freed of the copious water needed to disperse the fibrous slurry is generally transferred to a felt or fabric in a so-called press section where de-watering is continued either by mechanically compacting the paper or by some other de-watering method such as through-drying with hot air, before finally being transferred in the semi-dry condition to the surface of the Yankee for the drying to be completed.
The impact of the adhered web with the doctor blade is essential to impart to the paper web the properties which are sought by manufacturers. Of particular importance are softness, strength and bulk.
Softness is the tactile sensation perceived by the consumer as he/she holds a particular product, rubs it across his/her skin, or crumples it within his/her hand. This tactile sensation is provided by a combination of several physical properties. One of the most important physical properties related to softness is generally considered by those skilled in the art to be the stiffness of the paper web from which the product is made. Stiffness, in turn, is usually considered to be directly dependent on the strength of the web.
Strength is the ability of the product, and its constituent webs, to maintain physical integrity and to resist tearing, bursting, and shredding under use conditions.
Bulk, as used herein, refers to the inverse of the density of a tissue paper web. It is another important part of real and perceived performance of tissue paper webs. Enhancements in bulk generally add to the clothlike, absorbent perception. A portion of the bulk of a tissue paper web is imparted by creping.
The level of adhesion of the papermaking web to the dryer is also of vital importance as it relates to the control of the web in its travel in the space between the creping blade and the winder upon which a roll of the paper is being formed. Webs which are insufficiently adhered tend to cause poor control of the sheet with consequent difficulties in forming a uniform reel of paper. A loose sheet between the creper and the reel will result in wrinkles, foldovers, or weaving of the edges of the sheet in the rolled-up paper. Poorly formed rolls not only affect the reliability of the papermaking operation, but also the subsequent operations of tissue and towel manufacture in which the rolls are converted into the tissue and towel products.
The level of adhesion of the papermaking web to the dryer is also of vital importance as it relates to the drying of the web. Higher levels of adhesion reduce the impedance of heat transfer and cause the web to dry faster, enabling more energy efficient, higher speed operation.
However, the level of adhesion is not the sole factor determining product quality and manufacturing reliability. For example, some adhesives have been found to form a bond between the web and the doctor blade at the point of creping such that the web does not dislodge properly so that portions of the web remain adhered to the dryer and travel past the edge of the blade. This causes a defect in the web and often causes the web to break.
Further, while some amount of build up of adhesive on the dryer is essential, excessive build up or streaks can be formed with some types of adhesives. Streaks can cause differences in the profile of adhesion across the width of the dryer. This can result in humps or wrinkles in the finished roll of paper. A second doctor blade is often positioned after the creping blade in order to remove any excess creping adhesive and other residue left behind. This blade is referred to as a cleaning blade. Cleaning blades and creping blades must be changed at some frequency to prevent a streaky coating and loss of sheet control.
The term "doctorability" as sued herein refers to the relative ease with which the web is dislodged from the dryer without producing defects or requiring frequent changes of blades to prevent excessive build-up.
One important characteristic of a creping adhesive is that it be rewettable. "Rewettability", as used herein, refers to the ability of the adhesive film remaining on the heated drying surface to be activated by the moisture contained in the semi-dry tissue web when the web is brought into contact with the heated drying surface. A marked increase in tack is indicative of a high rewettability. Rewettability is important because only a portion of the drying surface is normally covered with adhesive on a given rotation of the Yankee dryer. The majority of the adhesion of the sheet to the dryer occurs by means of the creping adhesive deposited in previous passes.
There is a natural tendency of paper making web to adhere to the cylindrical dryer owing to the build up of deposits of water-soluble components of the paper web. These water soluble components form an adhesive film which is re-wet at the point of transfer of the web to the cylindrical drum. The needs for specific level and type of adhesion however has induced considerable activity among researchers in the field. Consequently, a wide variety of creping adhesives are known in the art. The use of animal glue, for example, has long been known.
In addition, Bates, in U.S. Pat. No. 3,926,716 incorporated herein by reference, discloses a process for improving the adherence of webs comprising the step of applying an aqueous polyvinyl alcohol.
As Bates points out, the needs for specific adhesion of the web to the dryer drum are particularly demanding when the paper making process is of the pattern densified category. Pattern densified webs are characterized by zones of relatively high density dispersed within a high bulk field, including more recent advances wherein the zones of relatively high density are continuous and the high bulk field is discrete. One method of preparing pattern densified tissue paper webs is referred to as through air drying. Pattern densified webs offer particular challenge to the adherence of the paper web to the rotating drying cylinder. This is because the web is only secured to the dryer cylinder in the high density areas. This challenges the adherence not only because of the lower surface area of contact with the dryer, but also because the web must be delivered to the rotating cylinder at a relatively higher level of fiber consistency owing to the poorer efficiency of the cylindrical dryer arising from the lower contact area. A web which has been dried to a relatively higher fiber consistency is more difficult to adhere to the dryer because a lower amount of water is available to rewet the adhesive film at the point of transfer of the web to the dryer.
In another example, Soerens, in U.S. Pat. No. 4,501,640 incorporated herein by reference, discloses an adhesive which comprises an aqueous admixture of polyvinyl alcohol and a water-soluble, thermosetting, cationic polyamide-epihalohydrin resin.
While a number of adhesives including these examples have been disclosed and are available, no single adhesive or adhesive blend has provided a satisfactory combination of doctorability, rewettability, and level of adhesion.
Therefore, it is the object of the present invention to provide an adhesive for creping tissue paper and a process for applying it which overcome these limitations by offering an improved level of adhesion while maintaining doctorability.
These and other objects are obtained using the present invention as will be taught in the following disclosure.